Thyristors, Sidactors and other four-layer devices are commonly used to provide overvoltage protection to circuits requiring the same. The Sidactor family of thyristors is a two-terminal device that has bidirectional current carrying capability, and is obtainable at many different breakover voltage values. When utilized in conjunction with telephone lines, for example, of the type in which 220 volt ringing signals are carried, a 250 volt breakover voltage Sidactor can be utilized to allow normal operation of the telephone line, but operate at 250 volts, or greater, in response to lightning strikes or power line crosses to thereby safely clamp the line to a very low voltage. This type of a device provides high surge current capabilities for protecting equipment from damage due to the extraneous voltages that may be coupled to the telephone line.
Many telephone circuits and equipment operate on a -48 volt supply voltage. To that end, Sidactors that operate at a nominal 64 volts are often utilized to protect such type of circuits. A nominally operating 30 volt Sidactor can be advantageously utilized to protect many 24 volt circuits, such as fire alarm and other systems, that are susceptible to extraneous voltages. It can be appreciated that the lines that generally require protection from damage due to extraneous voltages are often in environments where energy from lightning strikes can be induced into the lines, where high voltage AC circuits are in close proximity thereto, and for a host of other reasons.
While low-voltage digital lines, such as those driven by 5-volt TTL drivers are extensively employed in computerized and other equipment, such lines have not yet found a large application in outside installations. However, in view that computer networks and communications are increasing at a substantial rate, such low-voltage lines are being used in environments where overvoltage protection is required. Such overvoltage protection need not be due solely to lightning and power line crosses, but can be due to other standard voltages that are commonly found in indoor equipment.
It is well known in the thyristor and Sidactor field that the impurity level of a semiconductor wafer can be adjusted to thereby achieve a desired breakover voltage. It is commonly known that lightly-doped silicon substrates are characterized by high breakover voltages. As the doping or impurity level of the substrate is increased, the breakover voltage is reduced. It is also well known that the impurity level of a semiconductor material is inversely proportional to the resistivity thereof.
It has also been found that the use of buried regions in the semiconductor substrate facilitates the operational characteristics of Sidactors. See for example U.S. Pat. No. 5,479,031 by Webb. For example, if the Sidactor is constructed so as to have an a P-type emitter 18, an N-type base 16 and P-type substrate 12 or mid-region, a heavily doped P-type buried region 14 can be implanted between the base region 16 and the silicon substrate 12 to thereby reduce the breakover voltage. FIG. 1 is illustrative of this concept. Important advantages are achieved when the buried region 14 is directly beneath the emitter region 18, with the base region 16 material therebetween. Without significantly changing the impurity levels of the emitter 18, base 16 and substrate 12, the breakover voltage can be changed by simply changing the impurity level of the buried region 14. Moreover, in achieving breakover voltages from 250 volts down to 64 volts, the buried region need only be more heavily doped. In like manner, to achieve 30-volt breakover voltage devices, the buried region is required to be even more heavily doped.
As the impurity level of the buried region 14 increases, the junctions 20-26 formed between the buried region 14 and the base region 16 are displaced upwardly toward the emitter region 18. Indeed, as the doping level of the buried region 14 increases, the distance between the buried region-base junction 20 and the base-emitter junction becomes smaller and smaller. The reason for this is that the junction 20 is formed at a location in the semiconductor material where the donor states of one impurity are cancelled by the acceptor states of the opposite impurity. Stated another way, the junction of two semiconductor materials exists where the concentration of one region is equal to the concentration of the other region. The formation of a low breakover voltage Sidactor is not an elementary task.
It has been found that to fabricate nominal 10-volt breakover voltage Sidactor devices, the impurity level of the buried region must be so high that the buried region can often be effectively short circuited to the emitter region. In any event, even after fine tuning the processes so as to prevent short circuiting between the buried region and the emitter, the yield of workable devices is low, and thus such devices become costly.
Another problem attendant with the migration upwardly of the junction of the buried region is that the base region under the emitter becomes thinner. The distance of the base region between the emitter junction and the buried region junction defines, in part, a holding current (I.sub.h) parameter. The holding current is that current required to maintain an on-state of the device. A thinner base region adversely affects the ability to control a desired holding current.
Various other attempts have been made to make low breakover voltage thyristors. One endeavor involves a semiconductor design in which the breakover voltage occurs at the surface of the device. In other words, the concentration of the impurities at the surface of the device is controlled to achieve a low breakdown voltage.
From the foregoing, it can be seen that a need exists for a method and technique to fabricate low breakover voltage thyristor devices. Another need exists for a technique to fabricate low voltage thyristor devices where the breakover voltage is independent of the holding current. Yet another need exists for a thyristor device which can be reliably made with high yields, thereby reducing the cost of the devices.